THE BRIGHTEST MAIN SEQUENCE STAR IN M 67

Abstract

In color-magnitude diagrams of many globular and galactic clusters several stars are found located approximately on the main sequence but at considerably brighter absolute magnitudes than the vast majority of main sequence stars in the cluster. A few examples occur in the globular clusters M 131 and M 10,2 and numerous such stars are present in M 3.8·4 Galactic clusters having one or more stars lying above the turn-off point of the main sequence are NGC 752,5NGC 7789,e and M 67.7 All of these clusters also contain red giants. The presence of these early type stars raises serious problems in regard to the evolutionary interpretation of the color-magnitude diagrams. In the galactic clusters it may be argued that these stars constitute a fragmentary horizontal branch consisting of stars that were once red giants and are now evolving to the left in the diagram en route to the white dwarf region. A strong argument against this is the fact that in M 67 and particularly in NGC 7789 these stars appear as a faint extension of the main sequence and do not form a horizontal configuration. In the globular cluster M 3 they also form a faint extension of the main sequence above the turn-off point. Furthermore the globular clusters have well-populated horizontal branches that are entirely distinct from the stars in question. It is of considerable interest to study these stars in as much detail as possible. Star Fagerholm No. 81 in the galactic cluster M 67 has been chosen for investigation, since it not only is bright enough to observe spectroscopically at moderate dispersion, but also is located very considerably above the turn-off point of the cluster's well-populated main sequence. The evidence that star No. 81 is indeed a member of M 67 is very convincing. The proper-motion studies by van Maanen8 and Ebbighausen9 strongly suggest membership, as does the radial